This application claims the benefit of German Application 199 59 495.3, filed Dec. 10, 1999, the teachings of which are incorporated herein by reference.
Non-return valves are typically used in the field of injection molding when a plasticizing and injection screw rotates, to allow plastic melt, as it is transported from a material funnel by the screw with its screw channels, to reach the space before the screw, and, when the melt is injected into an injection molding tool, by translational motion of the screw, to prevent the melt from flowing back.
A plurality of non-return valves for injection molding machines are known. In terms of one of their characteristics, these can be divided into two structural forms; this characteristic relates to the design of the outer part of the non-return valve, which comes in contact with the inside wall of the preplasticizing cylinder:
The first group of non-return valves has an outside part which does not have a positive connection to the basic body of the non-return valve, and which is not constrained to rotate together with it.
The second group of non-return valves has an outside part which is constrained to rotate together with the basic body of the non-return valve.
Non-return valves belonging to the first group have the advantage that, when the screw rotates, no relative motion occurs between the cylindrical casing surface of the outside wall and the inside wall of the preplasticizing cylinder. However, they have the disadvantage that a high surface pressure appears on the mutually contacting axial surfaces, between the rotating part (basic body of the valve) and the non-rotating part (outside part), which can cause destruction of the non-return valve.
Non-return valves of the second group have the disadvantage that, when the screw rotates, relative motion occurs between the cylindrical casing surface of the outside part and the inside wall of the preplasticizing cylinder, thereby causing wear which attacks both the non-return valve and the inside wall of the preplasticizing cylinder. On the other hand, this design has the advantage that the axial surfaces are not attacked by wear, because the outside part does not move relative to the basic body of the non-return valve.
Both design forms have the common feature that the melt is prevented from flowing back by an axial motion of the components-namely the outside part relative to the basic body of the non-return valve-through the axial force acting on it through overpressure while the injection motion is beginning and, due to the axially disposed sealing surfaces, the melt flows back completely only if the sealing surfaces touch with simultaneous action of a high pressure force.
In light of these previously known designs of non-return valves, a non-return valve is provided which makes do with a minimum number of parts and which combines the respective advantageous properties of the above-cited two groups of non-return valves. At the same time, the respective known disadvantages of the two groups are avoided as much as possible. Finally, it is possible to close the lock securely but gently without great axial forces.
In accordance with aspects of the present invention, a non-return valve includes an outside part having at least two melt lines, which are not connected to one another, and the basic body has at least one melt line, such that the melt lines, of which there are at least two, of the outside body, together with the melt line of the basic body, in the first position jointly form a fluidic connection between the region of the screw channels and the region of the space before the screw, and such that the fluidic connection between the region of the screw channels and the region of the space before the screw is interrupted in the second position by a region of the basic body.
It is also within the scope of the invention that the outside part of the non-return valve together with the basic body of the lock can form a flow path for the melt-if the outside part and the basic body have an appropriate relative position, but that this flow path can be blocked if these two parts are moved relative to one another.
A further development specifies that the melt lines in the outside part are formed by bores. Several of these bores can be situated over the circumference of the outside part. The bores preferably are arranged equidistant over the circumference of the outside part. Furthermore, for rheological reasons it can be preferred that the angle (xcex1) between the axis of the bores and the rotation axis of the plasticizing and injection screw is between 30xc2x0 and 90xc2x0, preferably 45xc2x0.
The melt line of the basic body can be formed as a depression in the circumferential surface of the basic body.
The non-return valve of the present invention also cannot dispense with relative motions between the components. Consequently, it is proposed that the outside circumference of the outside part has a wear-resistant layer, in particular that it should be coated therewith. The like holds for the inside circumference of the outside part. Furthermore, the contact surfaces of the basic part, which the outside part can touch, have a wear-resistant layer, and in particular are coated therewith.
In one embodiment, the basic body has a tip, the basic body and the tip being formed integrally with one another. Furthermore, the basic body can be screwed rigidly to the plasticizing and injection screw. Finally, the outside part can have essentially the shape of a hollow cylinder.
The inventive design achieves various advantages such as the non-return valve can make do with a minimum number of parts. The lock can be produced correspondingly economically. Further, the contacting parts slide against one another with minimal speeds, as a result of which the non-return valve is correspondingly free of wear. The outside part of the non-return valve acts as a mixing part, which improves the homogeneity of the plastic melt. Furthermore, the non-return valve closes gently and firmly.